1. Field of the Invention
The present invention relates generally to the connecting together of threaded components. More specifically, the present invention relates to the threaded engagement of tubular components that are used in the drilling and a completion of wells used in the production of hydrocarbons.
2. Prior Art Setting of the Invention
Typical oilfield pipe includes drill pipe, casing and tubing. Relatively short pipe sections of 30 to 40 ft. or so in length are secured to each other to make a long string of pipe that is used in the drilling or construction of a well. The drill pipe is used to form a drilling string to deepen or work over the well. The casing pipe is used to case the wellbore to provide structural support for the well. The tubing pipe may be used as a production tubing string to bring the subsurface oil or gas to the well surface.
Each of the different types of pipe has a threaded end configuration specially designed to meet its intended purpose as it structurally secures and seals the pipe ends together. A common threaded connection is one in which a pin, or male threaded member, at the end of a tubular section engages a box, or female tubular member, at the end of the adjoining tubular section. The box may be formed as an integral part of the tubular section or it may be formed by securing a coupling to a pin end of a tubular pipe section.
The threaded connections of oilfield tubulars generally engage each other in an interference fit, in a shouldering fit, or in a combination of interference and shouldering fit. In an interference fit, a tapered pin and box are tightly wedged together as the pin is threadedly advanced into the box. The interference fit provides both structural and sealing connection between the pin and box. A shouldering connection engages an annular shoulder on the pin with an annular shoulder in the box. The threads of the shouldering connection provide the structure holding the pin and box together. The engaged shoulders provide the seal.
Threaded connections of both the interference type and the shouldering type tend to fail in areas of stress concentrations that occur during makeup and working of the connections. Connections of the shouldering type induce stresses in the engaged pin and box threads as a result of the load occurring when the shoulders are torqued together. Shouldering drill pipe tool joint connections typically fail in the threaded area within one or two threads from the base of the pin threads, primarily as a result of the stress concentration applied to threads as a result of the shouldering torque.
A variety of thread designs has been suggested to change the distribution of shouldering torque stresses along the engaged thread area of the pin and box of drill pipe connections to minimize the tendency of the connections to fail in the area of the base of the pin threads. Many of these thread designs distribute the torque load along the engaged threads by spacing the threads of the pin and box so that the load flanks of the threads are differentially loaded along the area of thread engagement. A common way of achieving this effect is to vary the thread pitch of the engaged threaded components to change the distribution of forces along the axial length of the threaded engagement.
U.S. Pat. No. 4,549,754, belonging to the Assignee of the present invention, and incorporated herein in its entirety for all purposes, addresses the problem of concentrated stress in the threads near the base of the pin by decreasing the pin thread taper relative to the box thread taper to achieve the effect of a changing thread pitch. At the fully made-up position, the patented design causes threads remote from the torque shoulder to become loaded before threads nearer to the shoulder are loaded. The patent also describes a special thread construction to reduce maximum stress in the thread roots.
U.S. Pat. No. 3,050,318 describes a technique for the distribution of makeup forces in which the pitch of the threads of either the pin or box connection changes gradually over the length of the thread development. The pitch difference is greatest at the middle of the thread development.
U.S. Pat. No. 4,842,464, relating to a bolt and nut configuration, distributes the axial load forces of a made up connection by forming a nut thread that is slightly different than the thread of the screw. The nut has a non-uniform pitch that increases progressively helically and axially along the length of the nut. The screw thread retains a uniform pitch along its entire length.
While the described prior art designs are effective in changing the load distribution in the threaded area of the assembled members, they can be relatively difficult to fabricate, requiring variation of more than one variable in the pin or box configuration or thread structure.